At present, the mobile communication network is mainly on the basis of Circuit-Switched (CS) such as Global System for Mobile communications (GSM) and Code Division Multiple Access (CDMA). Various operators establish relatively perfect and abundant service platforms on the basis of a CS network. The service presentation of the CS network needs the support of Mobile Switched Center (MSC) in a roaming area, which is not favorable for the introduction of a new service. IP Multimedia Sub-system (IMS) is an IP-switch-based service network. The service presentation of IMS is independent of the roaming area, which is favorable for the introduction of a new multimedia service. IMS supports a User Equipment (UE) to perform an IMS multimedia service via various packet-switching access networks.
Because the IMS network is a relatively complex network, the overall deployment of an IMS network can not be completed within a short period of time. Therefore, the CS network and the IMS network will coexist within a certain period. In this way, the operator may need to establish two service platforms for a new service in the CS network and the IMS network, which increases the costs of building and operating. To sum up, it is very necessary to provide a technology for a UE accessing an IMS service platform via a CS network to reduce the cost and facilitate the smooth evolution from the CS network to the IMS network. However, there is no such technology at present.